Electric motor and drive-controller design
3.7 Innovative drive scheme for DC series motors
Many DC brushed motor drive schemes for EVs use a DC shunt motor and it has been suggested that such a solution is the most appropriate5. This section investigates an alternative solution.
There are many railway locomotives which successfully use series wound motors and we hope to establish that indeed this is the best solution for electric vehicles.
3.7.1 MOTOR DRIVES: WHY CHANGE THE SYSTEM?
Because the system is already subject to change brought about by new requirements and developments. First, we have the introduction of sealed battery systems. These will permit much higher peak powers than hitherto possible and consequently will run at high voltages. 216 V DC is a common standard working with 600 V power semiconductors. Second, we have the introduction of hybrid vehicles. This will result in the need for drives and motors to operate for long sustained periods – previously batteries did not store enough energy. Third, the DC series motor has the right shape of torque–speed curve for traction, constant power over a wide speed range. Fourth, DC series field windings make much better use of the field window than high voltage shunt windings where much of the window is occupied by insulation. The series field winding is a splendid inductor for use in battery charging mode. Losses in series mode are significantly reduced.
B A
C D
TORQUE
344 Nm
1250 rpm 5000 rpm
45 kW
86 Nm
SPEED
Torque Speed Curve
An example specification is typified by the Nelco N200, Fig. 3.15(a), which compares with a 240 mm stack, Fig. 3.15(b):
Shunt field Series field
N = 227 N = 12
Hot resistance 7 Ω Hot resistance 0.014 Ω Watts 700 at 10 A Watts 500 at 189 A
So why hasn’t somebody attempted to use series motors in EVs before? They have for single quadrant low voltage systems but not on multi-quadrant, high voltage schemes. This account proposes a new control concept akin to vector control for AC machines. We will show how it is possible to achieve independent control of field current If and armature Ia, with very fast response, using a transistor bridge.
3.7.2 VEHICLE DYNAMICS AND MOTOR DESIGN
A vehicle represents a large inertia load with certain elements of resistance some of which increase with speed; see Chapter 8. For a small family car, mass = 1250 kg at 60 mph (26.8 m/sec) typical cruising speed. Windage accounts for 6 kW, rolling resistance 2 kW and brake drag 2 kW, a total of 10 kW in steady state conditions. Windage varies as the 3rd power of vehicle relative velocity with respect to the wind.
Kinetic Energy = 1/2 MV2, where M = mass = 1250 kg and V = velocity in metres/sec. So we have:
SPEED (MPH) 10 20 30 40 50 60 70 80
(m/sec) 4.5 8.9 13.4 17.8 22.3 26.7 31.2 35.6
KE (kilojoules) 12.5 49.5 111 198 309 446 607 792
What this illustrates is that recovered energy below 20 mph is small, consequently regeneration only matters at high speed. It also illustrates that the inertia load, not the static resistance, is the main absorber of power during acceleration.
3.7.3 MOTOR CHARACTERISTICS These are shown in the following table:
Voltage 216 V
Rated power 45 kW, 1250–5000 rpm Frame D 200 M- 4 pole with interpoles
Weight 170 kg
Fig. 3.15 Field windings: (a) shunt field machine; (b) 3 state strategy for series field machine.
(a) (b)
A
A
1 2 3
Cooling air forced, separate fan Winding, series field 245 A/216 V full load Efficiency at full load 85%
Field Resistance 10 milliohm, inductance 1.2 mH Armature Resistance 30 milliohm, inductance 260 mH inc. brushgear interpoles
Dimensions A = 490 mm, B = A + shaft, C = 335 mm, D = 350 mm; see Fig. 7.14 This illustrates that when the field current is strengthened in the constant power region, the armature voltage can be made to exceed the battery voltage and regenerative braking will take place. Below 1250 rpm plug braking must be used; however, the energy stored at this speed is small.
3.7.4 SWITCHING STRATEGY (SINGLE QUADRANT), FIG. 3.15
Figure 3.15(a) shows the arrangement for a 216 V, 45 kW shunt field machine with separate choppers for field and armature. There are some disadvantages with this scheme: (a) field is energized when not needed; (b) forcing factor of field is small – for a 45 kW shunt field, R = 7 ohm, I = 10 A nominal, L = 1.2 henries, t = 0.17 seconds; (c) when extended to multi-quadrant design two bridge chopper systems are needed if contactor switching is to be avoided; (d) extensive modifications are needed to provide for high power sine wave battery charging; (e) field power losses are significant (3 kW at max field).
Figure 3.15(b) illustrates the proposed new circuit which has a single 3 state switch: state (1) open-circuit; state (2) armature + series field; state (3) armature. So as an example, consider the following situation:
Full load torque at standstill
Field voltage for 245 A = 2 V Armature voltage for 245 A = 16 V
Fig. 3.16 Three state circuit expanded to 4 quadrant operation.
1
D1
so with 216 V battery:
D = 2/216 in state 2 D = 16/216 in state 3
The balance of the time will be off (D = duty cycle ratio for chopper).
It can be seen that by manipulating the relative times spent in each of the states, separate control of field and armature currents may be exercised.
When the speed of the motor exceeds the base speed (1250 rpm) the back-EMF is equal to the battery voltage and the switch henceforth operates only in states (2) and (3).
Let D = duty cycle for single quadrant chopper, then Vout/Vin = D, hence D2 (VB−5−KAωIf − IaRa − LadIa/dt) = If Rf + Lf dIf/dt and
VB− 5 = (KAωIf + IaRa + La dIa/dt) × (D2+ D3) where
ω = motor speed, rads/sec VB = battery voltage
KA = armature back-EMF constant V/amp/rad/sec (D2 + D3) D2 = duty cycle state 2
D3 = duty cycle state 3
Other symbols are self-explanatory.
3.7.5 MULTI-QUADRANT STRATEGY
Figure 3.16 illustrates the 3 state circuit when expanded to 4 quadrant operation: state 1 is all switches off; state 2 either Sl/S4 or S2/S3 on and state 3 is either Sl/S2 or S3/S4 on. As is clear, the third state is produced by having a controlled shoot-through of the transistor bridge. It may be considered that with two transistors and two diodes in series, voltage drops in the power switching path make the circuit inefficient. In fact with the latest devices: Vce sat for switches = 1.5 V at 300 A; Vf for diodes = 0.85 V at 300 A, giving a total drop = 4.7 V. So (4.7/216) × 100 = 2.3% power loss.
When the motor loses 15% this is a small deficiency. It represents 1.2 kW at full power. As the table illustrates in Fig. 3.16, all states of motoring and braking can be accommodated. The outstanding feature of this scheme is that the full power of the armature controller can be used to force the field, giving very fast response. From Fig. 3.16, it will be seen that the 4 quadrant circuit
Fig 3.17 4 quadrant circuit.
consists of a diode bridge Dl–D4 and a transistor bridge Sl–S4 (D5–D8). D9 acts as a freewheel diode when the transistor bridge is operated in shoot-through mode. Bridge Dl/D4 is required because the direction of armature current changes between motoring and braking. Control in braking mode is a two-stage process. At high speed the armature voltage exceeds the battery voltage and the battery absorbs the kinetic energy of the vehicle. At low speed the field current is reversed and plug braking of the armature to standstill is achieved via D9.
3.7.6 DEVICE PROTECTION IN A MOTOR CONTROLLER
Switches S1–S4 form a bridge converter and the devices require protection against overvoltage spikes from circuit inductances. The main factors are: (1) minimize circuit inductances by careful layout. The key element is the position of D9 and associated decoupling capacitor relative to Dl–D4; (2) fit 1 mF of ceramic capacitors across the DC bridge S1 /S4 plus varistor overvoltage protection.
Dl–D4 can be normal rectification grade components but D9 must be a fast diode with soft recovery. D5–D8 are built into the transistor blocks.
3.7.7 SINE WAVE BATTERY CHARGER OPERATION
With little modification the new circuit, Fig. 3.17, can be used as a high power (fast charge) battery charger with sine wave supply currents. The circuit exploits the series field as an energy storage inductor. Sl and D6 are used as a series chopper with a modulation index fixed to give 90% of battery volts. This creates a circulating current in the storage inductor. Switch S4 and diode D7 function as a boost chopper operating in constant current mode and transfer the energy of the storage inductor into the battery. Charging in this manner is theoretically possible up to 250 amps but will be limited by: (a) main supply available and (b) thermal management of the battery.
Fig. 3.18 Full circuit diagram of combined chopper/battery charger.
K4
CONTACTOR K1 K2 K3 K4 K5
MOTORING 0 C 0 C 0
BATTERY CHARGER C 0 C 0 C
Experience shows that charging at 30 amps is possible on a 220 V, 30 A, USA-style house air conditioning supply. Charging at greater currents will require special arrangements for power supply and cooling. One advantage of the scheme presented is that it may be used on any supply from 90 V to 270 V.
It is also possible to adopt the circuit for 3 phase supplies in one of two ways: (1) add an additional diode arm – this would produce a square wave current shape on the supply; (2) fit a 3 phase transistor bridge on the supply – this would permit a sine wave current in each line at a much increased cost.
3.7.8 POWER DIAGRAM FOR MOTORING AND CHARGING
Figure 3.18 presents the combined circuit diagram for motoring and battery charging. Reservoir capacitors and mode contactors have been added. The capacitors function as snubbers when running in motoring mode. As drawn, to adapt to battery charging, the battery plug is moved to outlet D and the mains inserted into plug B, alternatively contactors could be used to do the job. Battery safety precautions comprise: (1) the battery is connected via a circuit breaker capable of interrupting the full short-circuit current of a charged battery; (2) this circuit breaker is to contain a trip to disconnect battery by mechanical means only; (3) battery/motor/controller are each to contain
‘firewire’ to disconnect the circuit breaker; (4) circuit breaker is to be tripped by ‘G’ switch when 6G is exceeded in any axis.
3.7.9 CONTROL CIRCUIT IN MOTORING MODE
Figure 3.19 shows the block diagram of the controller for motoring mode. The heart of the system is a memory map which stores the field and armature currents for the machine under all conditions
Fig. 3.19 Control system in motoring mode.
TD1
(ROLLS OFF BELOW 10 mph)
MOTOR
Fig. 3.20 Block diagram of battery charging controller.
of operation. These demands for If and Ia are then compensated for in accordance with the battery voltage before conversion into analogue form, to be passed to operational amplifier loops which drive the modulators. Current feedback is provided by Hall effect CTs. The torque loop has input from two pedals and a feedback from a torque arm attached to the motor. Above the base speed there is no open circuit condition and the armature loop error is used to control the field.
3.7.10 CONTROL CIRCUIT IN BATTERY CHARGING MODE
The control circuit for battery charging is shown in Fig. 3.20. When the battery is below 2.1 V per cell and 40°C it is charged at the maximum current obtainable from the supply. Above 2.1 V/cell the battery is operated at reduced charging up to 2.35 V per cell, compensated at −4 mV/°C for battery temperature. This data assumes lead–acid cells.
As can be seen from the block diagram there are two separate loops for the buck and shunt choppers. The fast current loops stabilize the transfer function for changes in battery impedance.
The current limit function must be user-set in accordance will supply capabilities.
References
1. Hodkinson, R., Operating characteristics of a 45 kW brushless DC machine, EVS 12, Aneheim, 1995
2. Hodkinson, R., Towards 4 dollars per kilowatt, EVS 13, Osaka, 1996
3. Al’Akayshee et al., Design and finite element analysis of a 150 kW brushless PM machine, Electric Power Transactions, IEEE, 1998
4. Hodkinson, R., The characteristics of high frequency machines, Drives and Controls Conference, 1993
5. Hodkinson, R., A new drive scheme for DC series machines, ISATA 24, Aachen, 1994 6. Jardin and Hajdu, Voltage Source Inverter with Direct Torque Control, IEE PEPSA, 1987 Further reading
Alternative transportation problems, SAE, 1996
The future of the electric vehicle, Financial Times Management Report, 1995 Battery electric and hybrid vehicles, IMechE, 1992
Electric vehicle technology seminar report, MIRA, 1992 Electric vehicles for Europe conference report, EVA, 1991
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